It's the same stuff they use for self-cleaning windows, apparently. Prof Ryan told me that when light hits them they get excited, react with oxygen, and wash the pollution out of the air. It doesn't get rid of all the nasties from traffic, but it will eat up things called nitrogen oxides, which you can't see or feel but which have been linked to breathing problems including asthma. But it's not free of course. Professor Ryan comments: "If every banner, flag or advertising poster in the country did this, we'd have much better air quality. "It would add less than £100 to the cost of a poster and would turn advertisements into catalysts in more ways than one."He's also working on another idea, which is to add the nanoparticles to washing powder, so that your clothes soak up these poisons while you walk along.Two pairs of jeans could apparently clean up the nitrogen oxides from one car. And you wouldn't be able to see or feel anything different.Now for the words. Simon Armitage, who's professor of poetry at the university, has written a poem to go on the poster. It's called "In Praise of Air" (see box).The poster will be on display in Sheffield for the next year.